The present invention relates to a method of flattening an upper surface of an interlayer insulating film in a semiconductor device.
The demand of multilayer wiring is increasing with the progress of high-density construction of devices. In forming the multilayer wiring with a high density, it is important to flat form a surface of an interlayer insulating film on which the wiring is formed.
Various methods have been proposed to flatten an upper surface of an interlayer insulating film. Particularly in flattening the upper surface by forming the interlayer insulating film on a surface on which a plurality of projections each having a high aspect ratio due to the wiring are formed at intervals of about 100 .mu.m or more, for example, the interlayer insulating film is first formed by chemical vapor deposition or the like with a thickness larger than the height of the projections on the surface on which the projections are formed. Subsequently, a low-melting point compound (e.g., boron oxide (B.sub.2 O.sub.3)) is formed on the upper surface of the interlayer insulating film. Then, the low-melting point compound thus formed is melted by reflowing at a temperature of about 450.degree. C., for example, and is thereafter cooled to be solidified, thereby forming a flattening film of the low-melting point compound. Then, the flattening film and a part of the interlayer insulating film are etched back to thereby flatten the upper surface of the interlayer insulating film.
However, in forming the interlayer insulating film by the above method, it is necessary to carry out the reflowing of the low-melting point compound such as boron oxide by heating the same at the temperature of about 450.degree. C. Accordingly, in flattening the interlayer insulating film formed on an aluminum alloy wiring containing silicon, the wiring is also heated at the temperature of about 450.degree. C., so that the silicon contained in the wiring is deposited to increase a contact resistance. As a result, a reliability of the wiring is reduced. Further, it is difficult to form the flattening film having a completely flat upper surface by the reflowing.